Study: Low-Fiber Diets Could Be Messing With Gut Bacteria Diversity

A diverse population of microbes in your gut not only helps ward off pathogens, it’s also thought to protect against obesity and obesity-related health problems.

A new study out of Stanford University suggests that the low-fiber, highly processed diets so common in our industrial society may be depleting the diversity of our microbiomes (while we can’t digest fiber, it’s a major food source for gut bacteria). What’s more, because this deficiency appears to be heritable to an extent, the problem could be compounded over generations.

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To be fair, the research was conducted on mice. But study author Erica Sonnenburg believes it holds implications for humans. "There are very few ecosystems where low species diversity is a good thing,” she said in a statement. “There's no reason to think our gut is any exception.”

To test the hypothesis that a low-fiber diet lowers the diversity of the gut's microbiome, the researchers populated mice raised in a germ-free environment — meaning their intestines were microbe free — with bacteria from human donors. Some of the mice were then fed a diet high in fiber, while others were put on a diet that, while similar in protein, fat and calories, was extremely low in fiber.

Within weeks, mice in the low-fiber group exhibited weakened populations of bacteria strains while some strains appeared to entirely disappear. After seven weeks, the researchers moved these mice onto a high-fiber diet for an additional month. While some of the original bacteria species returned to full strength, a large percentage failed to recover entirely.

The researchers then bred mice raised on low-fiber diets. Fourth-generation baby mice — who were only exposed to the microbes from contact with their parents — exhibited severely depleted microbe ecosystems. Even after they were put on high-fiber diets, they lacked more than two-thirds of the bacteria species found in their great grandparents.

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Of course, unlike these mice, humans don’t exist in a vacuum — from subways to neighbors’ dogs, the world is full of bacterial sources. Still, the researchers worry that the sustained shift in our diets away from fiber-rich foods such as fruits and vegetables may be contributing to unhealthy shifts in our gut bacteria ecosystems.

“Over our history, humans have experienced major dietary changes from gathered to farmed foods during the agricultural revolution, and more recently to the mass consumption of processed foods in the industrialized world,” the study reads. “Each dietary shift was probably accompanied by a concomitant adjustment in the microbiota.”

Interestingly, the researchers found that fecal transplantation — in which fecal matter from mice with high-fiber diets was introduced into the intestines of fourth-generation low-fiber mice — fully restored bacterial diversity.

So far, it’s a potentially risky technique that has not been widely tested in humans. That’s changing, however — a randomized, clinical trial in which 20 obese participants ingest capsules containing fecal matter from lean, healthy donors for six weeks is slated to begin this year.